Parrotfeather Myriophyllum aquaticum (Vell.) Verdc.

California Pest Rating
Parrotfeather Myriophyllum aquaticum (Vell.) Verdc.
Saxifragales; Haloragaceae
Pest Rating: C |  Proposed Seed Rating: R

Initiating Event:

This plant was given a Q rating by the CDFA botany lab in 2015.

History & Status:                                  

Parrotfeather is originally from South America and has been introduced into several continents because of its popularity as a pond plant. Parrotfeather has established in bodies of water in California where it forms very dense mats of vegetation.

Parrotfeather is a perennial rooted aquatic plant that has both submersed and emergent forms. Branched stems grow up to 2 m long and 5 mm in diameter. Emersed branch networks form a horizontal pattern with extensive lateral branching followed by vertical growth. Emergent leaves are arranged in whorls of 3 to 6 leaves around the stem. The leaves themselves are divided, giving them a feathery appearance. Tiny, white flowers are produced on short stalks at the base of emergent leaves. In North America only female plants are known. Adventitious roots emerge from the stem nodes allowing the plants to grow vegetatively. The plants die back to rhizomes during the winter.

Official Control: Parrotfeather is listed as a noxious weed in Alabama, Connecticut, Maine, Massachusetts, Vermont, and Washington

Worldwide Distribution: Parrotfeather is native to South America. It has been introduced to Southeast Asia, Australia, New Zealand, South Africa, Japan and North America.

United States Distribution: Parrotfeather has been introduced to all states except the Interior West and the Central Northern States.

California Distribution:  Parrotfeather occurs in at least 27 California counties distributed throughout the state. As aquatic plants are under-collected and many water column weeds are superficially similar, it is likely more widely distributed than these collections indicate.

California Interceptions. Sent to CDFA by land managers.

Consequences of Introduction: 

1) Climate/Host Interaction: Evaluate if the pest would have suitable hosts and climate to establish in California. Score: 3

Low (1) Not likely to establish in California; or likely to establish in very limited areas.

Medium (2) may be able to establish in a larger but limited part of California.

High (3) likely to establish a widespread distribution in California.

Risk is High (3). Although open water habitat is sparse in CA, Parrotfeather has been able to spread to at least half the counties in the state. It has shown the ability to spread wherever appropriate habitat is available. Parrotfeather grows in slow-moving freshwater lakes, ponds, streams, and ditches; it responds well to high nutrient environments, where it grows in shallow water and on wet soil along shorelines; it is tolerant of moderate water fluctuations. It prefers freshwater but tolerates some salinity, so it could colonize wide areas in the CA Delta Region. Parrotfeather grows best in shallow water environments in which light reaches the bottom, but it can occur as a floating plant in the deeper waters of lakes. Parrot feather appears to prefer warmer, milder climates but is not seriously affected by frost. Once established, Parrotfeather persists despite variations in the environment.

2) Known Pest Host Range: Evaluate the host range of the pest. Score: 3

Low (1) has a very limited host range.

Medium (2) has a moderate host range.

High (3) has a wide host range.

Risk is High (3) as weeds do not require any one host, but grow wherever ecological conditions are favorable.

3) Pest Dispersal Potential: Evaluate the natural and artificial dispersal potential of the pest. Score: 3

Low (1) does not have high reproductive or dispersal potential.

Medium (2) has either high reproductive or dispersal potential.

High (3) has both high reproduction and dispersal potential.

Risk is High (3). Only female plants have been detected in North America, but like other water column plants, Parrotfeather grows and spreads extensively via vegetative means. Any small fragment can grow into a new colony if it reaches the appropriate shallow water habitat. As this plant is a popular pond plant, accidental and purposeful dissemination by hobbyists is a major pathway of spread.

4) Economic Impact: Evaluate the likely economic impacts of the pest to California using the criteria below. Score: 3

A. The pest could lower crop yield.

B. The pest could lower crop value (includes increasing crop production costs).

C. The pest could trigger the loss of markets (includes quarantines).

D. The pest could negatively change normal cultural practices.

E. The pest can vector, or is vectored, by another pestiferous organism.

F. The organism is injurious or poisonous to agriculturally important animals.

G. The organism can interfere with the delivery or supply of water for agricultural uses.

Low (1) causes 0 or 1 of these impacts.

Medium (2) causes 2 of these impacts.

High (3) causes 3 or more of these impacts.

Risk is High (3) as the dense growth can lead to flooding problems, impede irrigation, and obstruct recreational activities including boating, fishing, and swimming. It is a known weed of rice paddies, reducing yields. Parrotfeather has also been shown to provide excellent habitat for mosquito larvae; mosquitoes spread West Nile and zika viruses.

5) Environmental Impact: Evaluate the environmental impact of the pest on California using the criteria below.

A. The pest could have a significant environmental impact such as lowering biodiversity, disrupting natural communities, or changing ecosystem processes.

B. The pest could directly affect threatened or endangered species.

C. The pest could impact threatened or endangered species by disrupting critical habitats.

D. The pest could trigger additional official or private treatment programs.

E. The pest significantly impacts cultural practices, home/urban gardening or ornamental plantings.

Score the pest for Environmental Impact. Score: 3

Low (1) causes none of the above to occur.

Medium (2) causes one of the above to occur.

High (3) causes two or more of the above to occur.

Risk is high (3) as Parrotfeather has escaped cultivation and has spread into water bodies through intentional plantings and growth of plant fragments. The brittle nature of stems results in many fragments; these root easily in moist soils to establish new colonies. Plant fragments with their robust leaves and stems, and thick waxy cuticle, can survive periods out of water. Fragments can spread by currents, water fowl, and by boats. Parrot feather is present year round and may provide cover but has very little food value for wildlife. Its dense growth leads to competition with native vegetation and could impact sensitive species such as Bogg’s Lake hedge hyssop (Gratiola heterosepala), Lobb’s buttercup (Ranunculus lobbii), and Gambell’s watercress (Rorippa gambelii).

Consequences of Introduction to California for Uruguayan water primrose:

Add up the total score and include it here. (15)

Low = 5-8 points

Medium = 9-12 points

High = 13-15 points

6) Post Entry Distribution and Survey Information: Evaluate the known distribution in California. Only official records identified by a taxonomic expert and supported by voucher specimens deposited in natural history collections should be considered. Pest incursions that have been eradicated, are under eradication, or have been delimited with no further detections should not be included. Score: -3

Not established (0) Pest never detected in California, or known only from incursions.

Low (-1) Pest has a localized distribution in California, or is established in one suitable climate/host area (region).

Medium (-2) Pest is widespread in California but not fully established in the endangered area, or pest established in two contiguous suitable climate/host areas.

High (-3) Pest has fully established in the endangered area, or pest is reported in more than two contiguous or non-contiguous suitable climate/host areas.

Final Score:

7) The final score is the consequences of introduction score minus the post entry distribution and survey information score: Medium (12)


There is low uncertainty, as the plant has established in California and other states.

Conclusion and Rating Justification:

A widespread and potentially significant weed in CA of both natural wetlands and irrigation canals. Nevertheless, its wide distribution in CA and its availability in the aquarium and pond plant trade render it unlikely that regulation of its sale would be effective. It deserves a C rating, as it has spread widely through California. Eradication is impossible at this point in time.


Baldwin, B. G., D. H. Goldman, D. J. Keil, R. Patterson, T. J. Rosatti, and D. H. Wilken, editors. 2012. The Jepson manual: vascular plants of California, second edition. University of California Press, Berkeley.

Bossard, C.C., J.M. Randall, M.C. Hoshovsky, M.C. 2000. Invasive plants of California’s wildlands. University of California Press, Berkeley, California.

Consortium of California Herbaria:

Chambers, P.A., J.W. Barko, C.S. Smith. 1993. Evaluation of invasions and declines of submersed aquatic macrophytes. Journal of Aquatic Plant Management 31: 218-220.

USDA, NRCS. 2015. The PLANTS Database (, 18 December 2015). National Plant Data Team, Greensboro, NC 27401-4901 USA.

Responsible Party:

Dean G. Kelch, Primary Botanist; California Department of Food and Agriculture; 1220 N Street, Sacramento, CA 95814; Tel. (916) 654-0312;[@]

Pest Rating: C |  Proposed Seed Rating: R

Posted by ls